Data can be read out from a rotating optical disc by irradiating the optical disc with relatively weak light beam having a constant quantity and detecting the light that has been modulated by, and reflected from, the optical disc. On the other hand, in writing data on a recordable or rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the quantity has been changed according to the data to be written, and locally changing the property of a storage material film. Such optical disc read and write operations are described in Japanese Laid-Open Publication No. 52-80802, for example.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a readable and rewritable optical disc, a storage material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks, including spiral concave or convex portions, are arranged.
It should be noted that the depth or height of the pits, the depth of the concave portions of the tracks or the height of the convex portions of the tracks, and the thickness of the storage material film are all far smaller than the thickness of the optical disc substrate. For that reason, those portions of the optical disc, where data is stored, define a substantially two-dimensional plane, which will be referred to herein as a “storage layer”. Any optical disc includes at least one such storage layer.
To read or write data from/on a readable and rewritable optical disc, the light beam always needs to maintain a predetermined converging state on a storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the focal point of a light beam perpendicularly to the surface of a given optical disc (which direction will be referred to herein as a “focusing direction”). On the other hand, the “tracking control” means controlling the focal point of a light beam along the radius of a given optical disc (which direction will be referred to herein as a “tracking direction”) such that the light beam spot is always located right on a target track.
Next, a conventional optical disc drive will be described with reference to FIG. 1. The optical disc drive shown in FIG. 1 is an apparatus that can read and/or write data from/on a loaded optical disc 1 and includes a mechanism for rotating the optical disc 1 with a motor (not shown), an optical head 10 for irradiating the rotating optical disc 1 with light, and a signal processing and control section, which exchanges electrical signals with the optical head 10.
The optical head 10 includes a laser light source 11, a condenser lens 13, a polarization beam splitter 12, a focus actuator (which will be referred to herein as an “Fc actuator”) 14, a tracking actuator (which will be referred to herein as an “Tk actuator”) 15, and a photodetector 16.
A light beam, which has been emitted from the laser light source 11, is transmitted through the polarization beam splitter 12 and then focused onto the disklike optical disc 1 by the condenser lens 13. After having been reflected from the optical disc 1, the light beam is passed through the condenser lens 13 again, reflected from the polarization beam splitter 12 and then incident onto the photodetector 16.
When current is allowed to flow through the Fc actuator 14, the condenser lens 13, supported by an elastic body (not shown), moves in the focusing direction due to an electromagnetic force. On the other hand, when current is allowed to flow through the Tk actuator 15, the condenser lens 13 moves in the tracking direction.
The photodetector 16 outputs a light quantity signal to a focus error generator (which will be referred to herein as an “FE generator”) 30, a tracking error generator (which will be referred to herein as a “TE generator”) 40, a reflected light quantity detector 66, a wobble detector 83, and a disc type information reader 84.
The FE generator 30 functions as a focus error signal detecting section and generates an error signal representing the focusing state of the light beam with respect to the information layer of the optical disc 1 based on the light quantity signal supplied from the photodetector 16. The error signal is obtained, through computations, as a signal representing the deviation of the focal point of the light beam from the information layer of the optical disc 1 (which will be referred to herein as an “FE signal”). The FE signal is transferred to the Fc actuator 14 by way of a focus control filter (which will be referred to herein as an “Fc filter”) 31 and a focus control driver (which will be referred to herein as an “Fc driver”) 32, which function as a focus control driving section. The Fc filter 31 and Fc driver 32 perform phase compensation to get the focus control done with good stability.
In accordance with the FE signal supplied from the Fc driver 32, the Fc actuator 14 drives the condenser lens 13 in the focus direction such that the light beam is focused in a predetermined state on a certain information layer of the optical disc 1, which is so-called “focus control”.
The TE generator 40 functions as a tracking error detecting section and generates an error signal representing a positional relationship between the light beam spot on the optical disc 1 and the track (which will be referred to herein as a “TE signal”) based on the light quantity signal supplied from the photodetector 16. The TE signal is transferred to the Tk actuator 15 by way of a tracking control filter (which will be referred to herein as an “Tk filter”) 41 and a tracking control driver (which will be referred to herein as an “Tk driver”) 42. In accordance with the TE signal supplied from the Tk driver 42, the Tk actuator 15 drives the condenser lens 13 in the tracking direction such that the light beam spot follows the tracks, which is so-called “tracking control”.
In accordance with the signal supplied from the photodetector 16, the reflected light quantity detector 66 detects the quantity of the light that has been reflected from the optical disc 1 and outputs the reflected light quantity value detected to a disc type recognizer 85. In response to the signal supplied from the photodetector 16, the wobble detector 83 detects the amplitude of micro wobbling (which will be referred to herein as “wobble”) of the tracks on the optical disc 1 and outputs the amplitude value detected to the disc type recognizer 85. On receiving the signal from the photodetector 16, the disc type information reader 84 reads the optical disc information, which was written in advance on the optical disc 1, and transmits the optical disc information to the disc type recognizer 85.
Based on the signals supplied from the reflected light quantity detector 66, wobble detector 83 and disc type information reader 84, the disc type recognizer 85 recognizes the type of the given optical disc 1.
Suppose the reflectance of the optical disc changes with the type of the disc. In that case, even if the given optical disc 1 is irradiated with a light beam having the same intensity, the quantity of light reflected changes according to the reflectance of that optical disc 1. Accordingly, by comparing the quantity of the light reflected from the optical disc 1 with a predetermined level, the type of the given optical disc 1 can be recognized by the specific level of the reflectance of the optical disc.
Also, some types of optical discs may have the wobble but others not. Accordingly, if it is determined, by detecting the wobble amplitude of the given optical disc 1, whether or not the wobble is present on that optical disc 1, the type of the given optical disc 1 can be recognized.
Furthermore, information about a disc type may be stored on some optical discs. Thus, the type of the given optical disc 1 may be recognized by reading the optical disc information.
Hereinafter, another conventional optical disc drive will be described with reference to FIG. 2. In FIG. 2, any component, having the same function as the counterpart shown in FIG. 1, is identified by the same reference numeral as that used in FIG. 1 and the description thereof will be omitted herein.
The apparatus shown in FIG. 2 includes a disc type recognizer 67 functioning as disc type recognizing means. The disc type recognizer 67 recognizes the type of a given optical disc in accordance with a signal supplied from a reflected light quantity detector 66 and outputs a signal, representing the result of recognition, to a best optical wavelength selector 87.
The disc type recognizer 67 sends a low-level signal to an optical wavelength selector 90 while still recognizing the type of the optical disc, but sends a high-level signal to the optical wavelength selector 90 after having recognized the type of the optical disc.
A best optical wavelength table 86 stores information about the best optical wavelengths for multiple types of optical discs, from/on which this optical disc drive can read and/or write data. Also, the best optical wavelength table 86 provides the optical wavelength information for a best optical wavelength selector 87 and an initial optical wavelength selector 88.
The best optical wavelength selector 87 selects one of the best optical wavelengths in accordance with the recognition result of the disc type recognizer 67 and the optical wavelength information stored in the best optical wavelength table 86, and then outputs a signal, representing the optical wavelength selected, to the optical wavelength selector 90.
A selection index generator 89 supplies an index signal to the initial optical wavelength selector 88 so as to instruct the initial optical wavelength selector 88 to select the longest wavelength. In response to the index signal supplied from the selection index generator 89, the initial optical wavelength selector 88 selects the longest optical wavelength in accordance with the optical wavelength information stored in the best optical wavelength table 86 and supplies a signal, representing the wavelength selected, to the optical wavelength selector 90.
If the signal supplied from the disc type recognizer 67 is low, the optical wavelength selector 90 selects the optical wavelength, provided by the initial optical wavelength selector 88, for the laser light source 11. On the other hand, if the signal supplied from the disc type recognizer 67 is high, the optical wavelength selector 90 selects the optical wavelength, provided by the best optical wavelength selector 87, for the laser light source 11. In response, the laser light source 11, including multiple types of semiconductor laser diodes, for example, radiates a light beam having the specified optical wavelength.
Suppose an optical disc, from/on which data should be read or written at a long optical wavelength, has been loaded into an optical disc drive. In that case, if the optical disc is irradiated with a light beam having a short optical wavelength during a startup process, then the data may be lost from the optical disc, which is a problem. The lost data has a length corresponding to approximately one-fourth to one-half rotation of the optical disc. Accordingly, even if error correction were made, the lost data could not be correctible and could not be read at all. Among other things, a storage material film, which is optimized to a long optical wavelength, causes such a problem particularly easily because such a film absorbs a lot of light with short wavelengths. To overcome such a problem, a technique of using a long wavelength before the type of the given optical disc is recognized was proposed. A conventional optical disc drive of that type is disclosed in Japanese Laid-Open Publication No. 11-176073, for example.
Hereinafter, still another optical disc drive will be described with reference to FIG. 3. In FIG. 3, any component, having the same function as the counterpart shown in FIG. 1, is identified by the same reference numeral as that used in FIG. 1 and the description thereof will be omitted herein.
The apparatus shown in FIG. 3 includes a focusing instructor 77 and a control switch 78, which together functions as focusing means, and a search drive generator 79 functioning as search driving means.
The output of an FE generator 30 is supplied to an Fc filter 31 and the focusing instructor 77. The output signal of the Fc filter 31 is supplied to the control switch 83. In the initial state, the focusing instructor 77 sends a low-level signal to the control switch 78. However, after the FE signal supplied from the FE generator 30 has exceeded a predetermined level and decreased to less than a zero-cross point, the focusing instructor 77 sends a high-level signal to the control switch 78. The search drive generator 79 supplies a drive signal, which will move the condenser lens 13 toward the optical disc 1, to the control switch 78.
If the signal supplied from the focusing instructor 77 is low, then the control switch 78 passes the output signal of the search drive generator 79 to the Fc driver 32. On the other hand, if the signal supplied from the focusing instructor 77 is high, then the control switch 78 passes the output signal of the Fc filter 31 to the Fc driver 32.
Next, it will be described with reference to FIG. 4 how the optical disc drive shown in FIG. 3 performs a focusing operation. Portion (a) of FIG. 4 shows the output FE signal of the FE generator 30, portion (b) of FIG. 4 shows the output signal of the focusing instructor 77, and portion (c) of FIG. 4 shows the sources of drive signals to be selected by the control switch 78. In portions (a) through (a) of FIG. 4, the abscissa represents the time.
Once a startup operation is started with an optical disc loaded into the optical disc drive, the control switch 78 selects the drive signal supplied from the search drive generator 79 in the initial state. Then, the focal point of the light beam that has been converged by the condenser lens 13 is shifted toward the information layer of the optical disc 1. When the FE signal supplied from the FE generator 30 crosses zero after having exceeded a predetermined level FELVL, the output signal of the focusing instructor 77 changes from the low level into the high level. As of that moment, the control switch 78 selects the drive signal supplied from the Fc filter 31, thus turning the focus control ON.
In a multilayer storage optical disc with a plurality of information layers on which information can be stored, the light beam needs to be distributed uniformly to the respective information layers. For that reason, the greater the number of information layers, the higher the transmittance, but the lower the reflectance and absorbance, of each information layer should be.
Also, in a rewritable optical disc, written and unwritten areas of each information layer have mutually different reflectances. Accordingly, the quantity of reflected light detected changes depending on whether the spot of the light beam that has been radiated to recognize the type of the optical disc is located in an unwritten area or in a written area. Thus, the quantity of light reflected changes not only with the number of information layers the given optical disc has but also with the specific beam spot location on each information layer. For that reason, it is difficult to distinguish, just by the quantity of reflected light, several types of multilayer storage optical discs with different numbers of information layers from each other.
Furthermore, no matter how many information layers a multilayer storage optical disc has, each and every information layer thereof has track wobbles. Thus, it is difficult to distinguish, just by the wobble amplitude, several types of multilayer storage optical discs with different numbers of information layers from each other.
As described above, in a multilayer storage optical disc, as the number of information layers increases, the transmittance of each storage layer needs to be increased, and therefore, the reflectance and absorbance of each information layer both decrease. To compensate for such a decrease, as the number of information layers increases, the intensity of the light beam radiated from a laser light source needs to be increased. For that reason, if the type of the given optical disc 1 is recognized by reading optical disc information from the optical disc, then the intensity of the light beam radiated from the laser light source needs to be changed after the type recognition. However, if the intensity of the light beam is changed, then it takes a longer startup time because a learning operation for reading the optical disc information must be carried out again.
Furthermore, in a multilayer storage optical disc, the best light intensity of the light beam radiated from the laser light source changes with the number of information layers as described above. If a rewritable optical disc is exposed to a light beam with an excessively high intensity, then the information stored there may be altered. Also, if an optical disc is irradiated with a light beam, of which the intensity is higher than the best light intensity, before the type of the optical disc is recognized, then the information stored there will be lost to a non-correctible degree.
Meanwhile, to further increase the storage density of optical discs, the distance from the surface of an optical disc to an information layer thereof (which will be referred to herein as an “information layer depth”) tends to decrease. In a multilayer storage optical disc that has had its density increased in this manner, it is difficult to narrow the gap between the information layers so as to prevent the information layers from affecting each other. Thus, the variation in depth between the information layers increases. If the information layer depth changes at a greater percentage, then the spherical aberration produced on each information layer also changes more significantly. Hereinafter, this problem will be described with reference to FIG. 5.
FIG. 5 shows cross sections of two types of optical discs with mutually different information layer depths. Specifically, the optical disc shown on the left-hand side of FIG. 5 has a relatively deep information layer, while the optical disc shown on the right-hand side of FIG. 5 has a relatively shallow information layer. On each of these optical discs, a focus control is carried out such that the focal point of the light beam is located right on its information layer.
Suppose the light beam that has been converged by the condenser lens 13 produces the smallest spherical aberration with respect to the optical disc shown on the left-hand side of FIG. 5. In that case, the light beam is focused at a point on the information layer in the optical disc shown on the left-hand side of FIG. 5. However, if the information layer depth is different as in the optical disc shown on the right-hand side of FIG. 5, then the light beam is not focused at a point but a gap is created between the focal point of the light beam passing the inside portion of the condenser lens 13 and that of the light beam passing the outside portion of condenser lens 13, which is so-called “spherical aberration”. When such a spherical aberration is produced, the data that has been read from, or written on, the information layer has deteriorated quality. Thus, the spherical aberration needs to be adjusted with respect to the information layer on which the focal point of the light beam should be located.
In an apparatus of recognizing the type of a given optical disc based on the optical disc information that has been stored on the optical disc, the time it takes to finish the type recognition increases by the time to adjust the spherical aberration, thus extending the startup time of the optical disc drive unintentionally.
As described above, the magnitude of spherical aberration changes with the depth of the information layer. In addition, if the spherical aberration increases, the FE signal deteriorates and focusing becomes harder to accomplish.
In order to overcome the problems described above, an object of the present invention is to provide an optical disc drive, which can quickly perform a start-up process on a rewritable multilayer optical disc.